The present invention relates to a switching regulator for generating a DC voltage and a technique that is effectively applicable to a switching regulator required to have a good transient response, or a technique that is effectively utilized for, for example, a switching regulator provided in a system with a large variation in consumption current.
Lately, electronic devices in which a microprocessor is embedded as a system controller are increasing. The operating frequency of the microprocessor (hereinafter referred to as CPU) tends to become higher and higher and the maximum operating current increases, as the operating frequency increases. Meanwhile, portable or similar electronic devices incorporating the CPU often use a method of stepping up or down the battery voltage by a switching regulator to supply the operating voltage to the CPU. In these devices, it is carried out to deactivate whole the CPU or a partial circuit of the CPU when the CPU do not need to operate in order to reduce the consumption of the battery. In consequence, the range in which the current consumed by the CPU varies tends to be greater, as the maximum operating current increases. Due to this, a power supply device for supplying the operating current to the CPU is required to have a good transient response to the varying output current.
As a switching regulator having a good transient response, there is a switching regulator utilizing switching control called a hysteretic current-mode control method, for example, as disclosed in U.S. Pat. No. 5,825,165 (Patent Document 1). The hysteretic current-mode control switching regulator includes a current sense resistor connected in series to a coil to detect current that flows through the coil and an error amplifier that outputs a current proportional to a difference between a voltage (feedback voltage) that is a fraction of output voltage divided by a resistance voltage divider and a reference voltage. A hysteretic comparator compares an error voltage which is represented as the product of the resistance of a resistor connected between a code connecting the coil and the sense resistor and an output terminal of the error amplifier and the output current of the error amplifier to the output voltage. The switching regulator controls to maintain the output voltage at a constant level in the following manner. When a voltage drop across the sense resistor becomes more than “error voltage+hysteresis voltage,” a main switch allowing current to flow through the coil is turned from on to off and a synchronous circuit which acts to decrease the current flowing through the coil in sync with the main switch operation is turned from off to on. When the voltage drop across the sense resistor becomes lower than the error voltage, the main switch is turned from off to on and the synchronous switch is turned from on to off.
However, a problem associated with the above hysteretic current-mode control switching regulator is that additional power is consumed by the sense resistor connected in series to the coil. Since this power loss increases, as the maximum operating current increases, it would lead to a larger decrease in power efficiency in future application of the switching regulator. To decrease this power loss, it is conceivable to decrease the resistance of the sense resistor. However, an excessively small resistance of the sense resistor inhibits the monitored voltage from exceeding the hysteresis voltage of the comparator. This will result in a drawback such as a large output voltage ripple due to an unstable switching frequency.
The present applicant has previously proposed a switching regulator as an improvement over the above hysteretic current-mode control switching regulator in Japanese Unexamined Patent Publication 2004-064994 (Patent Document 2). A basic circuit of the switching regulator disclosed in this publication is shown in FIG. 8. As shown, a series circuit consisting of at least one resistor Rf1 and at least one capacitor Cf is connected in parallel with an inductor L1, and a resistor Rf2 is placed between a connection point of the resistor Rf1 and the capacitor Cf and a ground potential. An output voltage Vout added to a voltage proportional to a current flowing from the above connection point into the inductor L1 circuit is detected as a CR feedback voltage VCP. A reference voltage and the CR feedback voltage VCP are supplied to a hysteretic comparator circuit HCMP. The comparator generates a PWM control signal to switch a power output MOSFET M1 via a pre-diver. This MOSFET controls a current flowing through the inductor L1. It should be noted that a resistor DCR is connected as an equivalent series resistor (parasitic resistance) to the inductor L1 and this resistor is different from the current sense resistor as employed in switching regulator of the above Patent Document 1.
The output voltage Vout of the switching regulator described in the above Patent Document 2 can be expressed by the following equation (1): Vout=Vref−IL×DCR . . . (1). Here, IL is a load current. Hence, in a domain where the load current IL is larger, the equivalent series resistance component (parasitic resistance) DCR to the inductor L1 contributes to a large drop of the output voltage Vout and the load regulation deteriorates. A smaller supply voltage such as, for example, 1.3 V, tends to be used for the CPU as mentioned above and a hard disk drive system, whereas the maximum output current as large as 2 A is required. Allowable range in which a supply voltage of the power supply device varies is generally constant, for example, 3%. In a switching regulator for application of the above-mentioned low voltage output, an absolute value of the maximum allowable voltage becomes small and, therefore, the deterioration of the load regulation (IL×DCR) for the equivalent series resistance component DCR to the inductor L1 becomes non-negligible. For this reason, a circuit for preventing such load regulation deterioration, as is shown in FIG. 9, is also proposed. In this circuit, an error amplifier EA to which the reference voltage Vref and a fraction of the output voltage Vout divided by resistors Ra and Rb are input for comparison is additionally provided to generate a reference voltage to be input to the above comparator circuit HCMP.
[Patent Document 1] U.S. Pat. No. 5,825,165
[Patent Document 2] Japanese Unexamined Patent Publication 2004-064994